The present invention is directed to an improved process for the preparation of epoxy resins.
Methods mostly mentioned in the literature concerning the preparation of epoxy resin employ caustic alkali catalyst. The insolubility of the phenolate from the caustic-bisphenol-epichlorohydrin coupling process normally requires a large quantity of water and organic solvent to maintain a single-phase operation. Another preparation method (U.S. Pat. No. 3,221,032) proposes to eliminate the solubility problem attributable to phenolate formation by catalyzing the coupling of epichlorohydrin and polyhydric phenol with quaternary ammonium catalyst instead of the conventional caustic catalyst. The dehydrochlorination is then carried out at a relatively high temperature in the presence of epichlorohydrin. To increase epichlorohydrin yield, it has been proposed (U.S. Pat. No. 2,943,096) that removal of the unreacted epichlorohydrin from the coupled intermediate be performed prior to dehydrochlorination of the resin chlorohydrin which would then be carried out in a mixture of solvents other than epichlorohydrin. The disadvantages of this process are undesirable product viscosities and the need to use a mixture of water-soluble and water-insoluble solvents to achieve dehydrochlorination at a rapid enough rate to be practical. Additionally, product viscosity control in the 9,000-10,000 cps range has previously required additional process steps (i.e., addition of seed resin, double charging of reactants to the coupling reaction, and post addition of a polyhydric alcohol following dehydrochlorination). The present invention describes a process which significantly eliminates the disadvantages and retains most all of the advantages of a non-caustic catalyzed process. Consistent product viscosity and adjustment without additional process steps in achieved by control of the unreacted phenolic OH content in the coupled intermediate prior to dehydrohalogenation, thus minimizing and controlling viscosity building oligomers and high resin bound chloride species. The remaining required product oligomers contributing to viscosity control are formed during dehydrochlorination without significant additional buildup of bound chloride. This processing method allows for flexible viscosity control while maintaining a low chloride product.